A few weeks back, AMD quietly released a couple of new graphics cards in the Radeon HD 5500 series. For a while there, starting with the Radeon HD 5870 in late September of last year, it seemed like AMD was releasing a new series of graphics cards aimed at different price points, practically every few weeks. Things slowed down a bit once the company had a complete top-to-bottom line-up of DX11 and Eyefinity capable cards from prices ranging from about $49 to $600, but obviously AMD wasn’t quite done beefing up the Radeon HD 5000 series.
The recently released Radeon HD 5500 series cards differed from their predecessors in only one meaningful way—they were equipped with GDDR5 memory. The original 5500 series cards sported GDDR3 or GDDR2 memory. The move to GDDR5 allowed AMD to crank the memory clock speed up a bit, which would have a positive impact on overall performance...

"These new algorithms attempt to find order in what we all see as the
randomness of ray tracing and they attempt to drastically increase the
memory locality for efficiency reasons and the founders decided that
required a custom co-processor. But they seem to have played the game
intelligently by continuing to utilize hardware where it is most efficient:
the Caustic card will handle only the operations that modern components are
inefficient at yet they still leverage the power of the GPU for pure shading
horsepower."